It may not be ALL the hardware, but the network card itself... Before you make yourself crazy, try a different one... Or even try swapping them in the setup, and then comparing. You should then maintain your external connection, and regulary lose your internal.
Neither is acceptable, but if you have to reboot to re-connect with the DSL router, the clients couldn't access it then either. Once you have them switched and a condition where the clients can't get outside access, test your internet connection.